1. Field of the Invention
This invention relates to an objective lens for an optical disc and an optical system thereof.
2. Description of the Related Art
FIG. 1 shows a general optical system used for detecting information recorded in an information-recording medium such as an optical disc. Namely, a ray of light emitted from a light source LD is collimated into a parallel ray of light by a collimate lens CL and the parallel light is converged on an information surface of an optical disc D by an objective lens OL to optically read out information of the optical disc. The light reflected on the information surface of the same optical disc D is again introduced into the objective lens OL to be collimated into the parallel ray of light. The light reflected by a half-mirror is converged into a detecting element PD by the collimate lens CL. An optical signal is converted into an electric signal by the detecting element PD, thereby converting the information of the optical disc into the electric signal. Incidentally, the system shown in FIG. 4 is referred to as "Infinite System".
Also, as shown in FIG. 2, the light emitted from the light source LD is directly converged on the information surface of the optical disc D by the objective lens OL, the information of the optical disc is optically read out. The light reflected thereon is again introduced into the objective lens OL, and the light is directly converged into the detecting element PD through the half-mirror M. Incidentally, the system shown in FIG. 5 is referred to as "Finite System".
In taking into consideration miniaturization, lightening, reliability and cost performance, it is advantageous to constitute the optical system by using the finite system in which the number of parts is fewer than that of the infinite system in the two optical systems mentioned above. Typical one of such optical systems is disclosed in Japanese examined Patent Publication No. Hei 4-28282.
However, the conventional objective lens disclosed above suffers the following problems.
In taking into consideration a convergence performance onto the information surface of the optical disc D, it is common for the radius of curvature of one side of a single lens into which the light is introduced to be smaller than that of the other side thereof from which the light is emitted. The conventional objective lens is shown in FIG. 3.
Moreover, all of the objective lenses disclosed in each of the examples in the Japanese examined Patent Publication No. Hei 4-28282 are made in such a manner that a radius of curvature r1 of the light introduced side is smaller than a radius of curvature r2 of the light emitted side, as shown in TABLE 1.
TABLE 1 ______________________________________ r.sub.2 r.sub.1 .vertline.r.sub.2 /r.sub.1 .vertline. ______________________________________ Example 1 -1.31979 0.63363 2.083 Example 2 -2.01409 0.69784 2.886 Example 3 -1.8412 0.71255 2.584 Example 4 -2.77870 0.70448 3.944 Example 5 -1.90409 0.77168 2.467 Example 6 -5.04983 0.95664 5.279 Example 7 -1.08514 0.69588 1.559 Example 8 -2.36833 1.11492 2.124 Example 9 -0.87863 0.74996 1.172 Example 10 -1.51637 0.60545 2.505 ______________________________________
However, an optical disc device is required to converge the light emitted from the light source LD on the information surface of the optical disc D by the objective lens OL. Furthermore, in the optical disc device, it is required that the converged ray of light be reflected on the information surface in the same optical disc D to again converge the light into the detecting element PD through the objective lens OL.
Therefore, if the convergence performance onto the detecting element PD is low (even if the convergence performance onto the information surface of the optical disc D is high), the system suffers from the problem that the information recorded on the optical disc can not be correctly read out.
Moreover, recently, because the optical system for the optical disc has to be miniaturized in accordance with miniaturization of the optical disc device, the convergence performance onto the detecting element PD is required to further be improved.